Wrath of Khan at 35: Entering the Mutara Nebula

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35 years ago this week, Captain Spock uttered very important words toward the end of Star Trek II: The Wrath of Khan.

I don’t mean "The needs of the many outweigh the needs of the few … or the one,"* or "I have been and ever shall be your friend."

Nope. It's before that. As you may remember — and you should — after numerous skirmishes, Kirk goads Khan into pursuing him after leaving the space station Regula One. Enterprise is on full impulse, heading away from the station, when Spock says, "We are now entering the Mutara Nebula."

What follows is still to this day one of the best starship-to-starship battle scenes ever filmed. Tense, tightly paced, beautifully directed and scored. And it started with that line.

But ... Does it make sense? Is there really a moment when you've entered a nebula? And — as cool as it was — would it really look the way it's depicted in the movie?

Enterprise and Reliant in the Mutara Nebula. Credit: Paramount Pictures

A brief digression, though: Khan is one of my favorite movies. I mean, of all time. I love this flick so much. I could rhapsodize on it at length, but (unlike the acting in it) I'll hold myself back a bit here. Despite that, there's some stuff, sciencey stuff, that holds up well to scrutiny even all these years later. But then, there's other stuff that, um, maybe doesn't so much. So before you lock me in a room with a wild mugato, bear in mind my fondness for this iconic movie.

So, first up … Let's start at the beginning (after all, for everything there is a first time): What's a nebula?

Generally speaking, it's a big cloud of gas floating in space. Nebula actually means "cloud" in Latin! But this is a bit too general; there are many kinds of nebulae. I covered all this in an episode of Crash Course Astronomy that'll get you up to speed:

Video of W8UI7F43_Yk

So how does the Mutara Nebula stack up? Mind you, this is a fictional nebula, so don't bother trying to find it on any star chart; it is not down on any map; true places never are. It doesn't actually exist. But it does have some features of a real nebula; from a distance (say, in the background as Enterprise and Regula 1 hang in the foreground) it looks diffuse, and appears blue and red.

First, the shape: The fuzzy nature of the nebula is dead on. These are not solid objects like planets, with a firm boundary. They're clouds, and seriously rarefied ones. A thick nebula has a density of maybe a few thousand atoms per cubic centimeter; compare that to 1019 (10,000,000,000,000,000,000 or 10 quintillion) atoms of air in a similar volume at Earth sea level. They're barely more than a vacuum! The reason they look so obvious is because they're so big; over all that volume even a few thousand atoms per cc adds up.

Also, nebulae just sorta fade away with distance, getting thinner and less dense. In many cases, this happens over many billions or tens of billions of kilometers.

If the Mutara Nebula were an enormous pillow a light years across, this might work. Note how far Saavik, Sulu, and David are thrown forward. Kirk is a rock. Credit: Paramount Pictures

And so, reluctantly, even though it's depicted wonderfully, I must give Wrath of Khan a demerit: Right after Spock says they're going in, the Enterprise slows suddenly, like they've rammed a gigantic marshmallow. In that scene, in fact, we see everyone on the bridge of the Enterprise suddenly flung forward due to the ship's sudden deceleration (well, almost everyone: Kirk actually doesn't move at all; apparently Shatner missed his cue and didn't lunge forward). That wouldn't really happen because the nebula doesn't have a sharp enough edge. Even at the (sublight) speed the Enterprise was traveling it would take several minutes (or really hours) to feel any real deceleration due to drag.

Maybe Shatner had it right. Anyway, it makes for a nice dramatic cue, but in real life it wouldn't happen.

The Mutara Nebula is both an emission and reflection nebula. Credit: Paramount Pictures

The colors, though, are dead on. In the image above, you can see the left side of the nebula is red. It also has lots of stars embedded in it. That's perfect! If those stars are hot and massive, they'll energize the nebula. They emit ultraviolet light, which has more energy than visible light (the kind of light we see). UV light has enough energy to strip electrons off of hydrogen atoms — the predominant constituent of nebulae. This is called ionization. When the electrons then recombine with the hydrogen atoms, they emit light, preferentially in the red part of the spectrum. That's why these nebulae glow red … and we call them emission nebulae.

On the right, the Mutara Nebula is blue. That's good too! Nebulae tend to have lots of dust in them, tiny grains of silicates (rock) and very complex long-chain carbon atoms. When this material is farther from hot stars it can scatter light (bounce light off molecules in semi-random directions) without becoming ionized. Due to the physics of scattering, blue light is scattered more efficiently than red light. Also, the stars are hot and tend to be blue, and so the light we see from these nebulae is blue as well. We call these reflection nebulae.

So the Mutara Nebula is shown pretty accurately. Many nebulae are red near the stars and bluer farther out. If I wanted to fuss — and to be honest I don't, but it brings up a cool scientific point I can't resist — there should be far fewer stars seen in the denser, dustier part of the nebulae, since the dust blocks the light from stars behind it. Also, those stars should appear redder, since their blue light is scattered away. You can see this beautifully in one of my favorite nebulae, Barnard 86:

B86 is small as such things go, a mere 2 trillion kilometers wide (most nebula you see in photos are several light-years across, where a light-year is 10 trillion km). It's very dense and dusty, and in the visible light image (left) it blocks the light from stars behind it. But note the stars near its edge; they look red, because red light isn't scattered as efficiently, so at the less dense edges of the nebula the red light from stars can still get through.

Now look at the picture on the right, which shows B86 in visible light (shown as blue) but also in far infrared (shown as red). That infrared light is scattered even less, and can penetrate through even the denser parts of the nebula. So we can see the stars behind it! About a thousand stars show up, stars which would be invisible if we only looked in visible light.

B86 is dense and cold (its temperature is only 16 Kelvin, or -460°F!). Its gravity wants it to collapse, but its meager warmth generates just barely enough pressure holds it up. It's in a precarious balance. If it cools a bit more, or something perturbs it (maybe colliding with another cloud), it'll collapse. This is how stars form! They condense out of nebulae like these, or sometimes in gigantic versions of B86 called giant molecular clouds.

If there's enough gas, sometimes these newborn stars are massive and hot, and blow out a fierce wind of particles. They can carve huge cavities into the cloud, and if this happens near the side of the cloud they can create blowouts, like popped bubbles. Their light can then transform that part of the cloud into emission and/or reflection nebulae. The magnificent Orion Nebula is probably the most famous of these.

B86 is too small for this, though. When it collapses, it will have just enough mass to form either a single star like the Sun, or a handful of lower-mass red dwarf stars.

Which brings us back to the Mutara Nebula. It's lovely, but at this point in the movie it doesn't have long to exist. After the battle, Khan sets off the Genesis Device. Spock sacrifices his life to fix the warp drive, and Enterprise zooms off. The device explodes, the nebula disappears, and the Genesis planet and star are formed.

The science here is a tad dicey. It's not clear how Genesis (designed to remake worlds, not create them) makes a planet form. Given the red color of the nebula, maybe there were already nearby stars, and the planet happened to form near one. Given how big space is, that seems unlikely, though. So it seems that a star formed with the planet as well. Let's say the device was programmed to do that if it had sufficient material.

This is more than a cool effect; concentric light echoes are common in dense environments in space when a sudden flash of energy is released. Credit: Paramount Pictures

That explains why the nebula disappears. It was the raw material to form the planet and star! I'm willing to go with that. Interestingly, as Enterprise warps off, we see concentric rings of material around the Reliant explosion. This happens in real life too! As the flash from the explosion expands, it can light up sheets of material around it. When an expanding sphere intersects a flat sheet, we see that as a circle (think of a piece of paper slicing through a ball; what you get is a circle). This is called a light echo, because it's similar in nature to a sound echo; the light is reflected off material. One of the most famous is the light echo around the star V838 Monocerotis:

In this object we think two stars merged, creating a huge flash of energy. The stars were surrounded by thick clouds of dust, and over time, as the light from the flash moved outward, we see more distant material lit up by it. A more classic example, and one that looks much more like what we see in Wrath of Khan, are the circular light echoes seen after Supernova 1987A blew up.

Perhaps the most surprising thing of all about a nebula is that, if you were inside it, counterintuitively you might not see it at all! All that nebulosity and color are obvious through a telescope, but if you were inside the nebula that light would be spread out all over the sky. That substantially dims it, making it difficult to see. Some things, like shock waves and thread-like filaments of compressed gas would be easier to see, but overall the view from inside a nebula may not be as exciting as depicted. Ah well.

One more thing: During the battle scene, we see a lot of weird goings-on in the nebula. Swirling clouds, flashes of light moving like a wave through the gas, gas moving like fog over a mountain, sparkling curtains of material changing shape and brightness, regions seen as pink and purple … while we've never necessarily seen things just like these in nebulae, given the scales of those scenes the features aren't that big, maybe a dozens or hundreds of kilometers across, so from Earth they're invisibly tiny.

So, overall, I think the depiction of the Mutara Nebula in Wrath of Khan is a mixed bag; it gets some things right, and other things wrong, as so many movies do. But that detracts nothing from it, in my opinion, and I always love the chance to use something like this to show you the real Universe.